Sound is often described as a moving pressure wave. A sound source is a moving object that creates a change in air pressure as the object moves. Air particles that are adjacent to the sound source begin to move as a result in the change in air pressure. Each subsequent moving air particle creates a change in air pressure to other adjacent air particles resulting in a moving pressure wave. Microphones are devices that are arranged to measure variations in air pressure and convert those pressure variations into electrical signals.
Many different technologies are commonly used to accomplish the conversion of air pressure into an electric signal. One particular type of technology is referred to as a condenser microphone. A condenser microphone is essentially a capacitive device, where one plate of the capacitor moves in response to sound waves. The movement in the plate results in a change in capacitance value of the capacitor. The capacitive changes are amplified to create a measurable signal that is typically an analog voltage. Condenser microphones usually need a small battery to provide a voltage across the capacitor plates.
An electret microphone (ECM) is a variant of a condenser microphone. In one type of ECM, a permanently charged diaphragm is placed over a conductive metal back-plate. In another type of ECM, a charged back-plate is used instead of a charged diaphragm. Recently, ECMs have been combined with various electronic circuits to provide digital output signals instead of an analog voltage.
An example digital ECM is illustrated in FIG. 1. The digital ECM includes a diaphragm that is separated from an electret by an air-gap. The Electret includes a back-plate that is electrically connected to a power supply (VCC). The variations in the capacitance of the ECM are processed by an integrated circuit. The integrated circuit includes a preamplifier that amplifies the signal levels from the ECM, and an analog-to-digital converter (ADC) that converts the amplified signal levels to a digital signal. The integrated circuit requires a power signals (VCC, GND), and a clock signal (Clock-In), to provide the digital output (Data-Out).